EFFECTS OF TIRE PROFILE WITH LEAN ANGLE

MotoSPEC v5.5 introduced analysis of lean angle and tire profiles in the PRO version of MotoSPEC.

Please see the post 'SPECIFYING TIRE PROFILES' for an explanation of tire profiles.

---

The image below shows the location of the two contact points in front views of the motorcycle, in the vertical condition and also as the motorcycle leans.  In the vertical position, the points of the tire profiles on the center line of the bike are in contact with the ground.

With lean, the chassis pitches forward due to the difference in tire profiles, because the front profile is less wide than the rear profile.  The profile points on the center line of the bike are no longer aligned.

The Contact Points are the points on the profiles that are tangent to the ground.
Deformation of the tires is not considered - the contact is simplified to a point.

The front braking force acts perpendicular to this view thru the Front Contact Point.
The drive force applied at the rear tire also acts perpendicular to this view thru the Rear Contact Point.

[CLICK TO ENLARGE IMAGE]

---

TRAIL WITH LEAN

The image below shows a view of the motorcycle cornering with lean, viewed from the inside of the curve.  Some geometry of the bike has been omitted for clarity.

The steering geometry is shown in the center plane of the front wheel.

This also shows how the contact patch is offset from the steering axis, and how the trail braking forces cause a torque tending to turn the front wheel into the corner.

[ CLICK TO ENLARGE IMAGE ]

---

GEOMETRY WITH LEAN

The image below is looking towards the center plane of the leaned motorcycle, showing the front steering and rear anti-squat geometry as affected by the reduced tire radii with lean angle.

The angle of the anti-squat line is reduced, caused by the smaller rear tire radius. The anti-squat angle is also affected by reduced swingarm angle, caused by the forward pitch of the chassis with lean.

The trail is reduced because of the smaller front tire radius with lean, and also the forward pitch of the chassis with lean.

The image below also shows how the Normal Trail is the direct lever that the lateral forces at the contact patch act on.

[ CLICK TO ENLARGE IMAGE ]

---

MotoSPEC DISPLAY OF LEAN ANGLE RESULTS

The image below shows the geometry of the motorcycle, as displayed in MotoSPEC in the following conditions:

1. Topped out, for reference
2. Mid-Corner potentiometer readings without lean angle.
   
3. Mid-Corner potentiometer readings WITH lean angle.
   

 
The geometry of the third column (red) shows the swingarm angle is reduced by the forward pitch due to lean.  The anti-squat angle is further reduced due to the smaller rear tire radius.

At the front, the rake is reduced due the forward pitch with lean, and the trail is further reduced due to the smaller tire radius in the leaned condition.

[CLICK TO ENLARGE IMAGE]